The influence of environmental factors on microstructure of otoliths of three demersal fish species caught off Namibia

Abstract

The microstructure of otoliths of the hakes Merluccius capensis and M. paradoxus and kingklip Genypterus capensis was studied by means of a scanning electron microscope. The otoliths of all three species displayed distinct daily growth rhythms. Starting from the core, regular microstructural patterns of deposition produced distinct increments which could be enumerated from the rugose surface of prepared otoliths. During the early stages of growth, deposition of the increments was daily, but this regularity was lost with age. Checks or discontinuities, which may record periods of perturbation or stress of the fish, are frequent in both species of hake and in kingklip. In the two Merluccius species the growth increments contain sub-daily units and patterns of increment deposition, probably dependent upon the combined effect of temperature variation and migratory behaviour. Sub-daily units were not found in the increments of kingklip otoliths. As kingklip always remain close to the bottom, the absence of sub-daily units may be related to the day-to-day stability of the benthic environment combined with their infrequent feeding periodicity. The greater intensity of the seven-day cycles in the rings laid down in winter would seem to indicate that such cycles may be related to environmental fluctuations produced by changes in upwelling intensity. © 1987 Taylor & Francis Group, LLC.